Particle-optical apparatus with focused ion beams are applied in the semiconductor industry for the purpose of processing wafers with focused ion beams. To this end, an ion source is imaged onto the wafer into a so-called ion spot. The processing speed with such ion sources is limited by the ion current density in this ion spot. A high ion current density is achieved by focusing a bright ion source into the ion spot. It is desirable to use ions which do not remain behind in the processed wafers, such as noble gas ions.
A gas ion source for a particle optical apparatus is described in U.S. Pat. No. 7,772,564, hereby incorporated by reference, which is assigned to FEI Company, Inc., the assignee of the present invention. The gas source comprises a diaphragm wall, at a first side of which diaphragm wall is located a gas that is to be ionized, at a gas pressure of, for example, 0.2 bar. At the other side of the diaphragm wall is located vacuum, or at least a space with lower gas pressure. In the diaphragm wall, an exit diaphragm is fitted, through which exit diaphragm gas flows out into the vacuum. Electrons generated by an electron source at the vacuum side of the diaphragm wall are accelerated by a first electric field, the acceleration field, and focused by an electron lens, whereby the electron focus is located just before the exit diaphragm on the vacuum side of the diaphragm wall. As a result of collisions between the electrons in the electron focus and the emerging gas atoms, gas ions are now formed in an ionization volume that is thus in the direct vicinity of the exit diaphragm. The volume of the ionization volume is determined by the region in which, concurrently, a high electron density and also a high gas density occur. The ions are extracted from the ionization volume with the aid of a second electric field, the extraction field, and can then be imaged and manipulated with the aid of particle-optical means known in the prior art.
Gas sources, such as the source described in U.S. Pat. No. 7,772,564, can maintain a high brightness by keeping the ionization volume small, seeing as the brightness is otherwise limited by plasma and space-charge effects. Currently, electron sources, such as sources employing field emitters, Schottky emitters or Carbon Nano Tubes, are often used when there is a need for high brightness electron sources. These sources have small electron-emitting surfaces. As known to the skilled artisan, these sources should be imaged by optics with small aberrations, especially when a relative large current in the image is to be obtained. In some applications, electrons are provided with a “sideways injection” into the ionization volume such that the electrons are applied perpendicular to the field extracting the ions from the ionization volume.
The gas source in U.S. Pat. No. 7,772,564 is limited to provide a single ion species. However, there are certain applications when the use of multiple ion species is desired due to different characteristics of the ion species. For example, a light ion is well suited for microscopy because of its low sputter yield, and a heavy ion with a high sputter yield is well suited for milling applications. Choosing an ion species with specific chemical properties can also greatly enhance applications such as beam chemistry or analysis.
Moreover, it is also desirable to change between different ion species quickly and efficiently while operating the particle optical apparatus in order to tailor to certain applications. Current systems require users to change the single gas source entirely and replace it when a new ion species is desired, which is time consuming and requires processing of the sample to be interrupted, thereby causing processing errors such as positional errors or reaction errors due to excess time.
Prior art liquid metal ion sources (LMIS) employ a mass filter capable of separating species of a common source. However, LMIS typically do not achieve the same brightness levels as a gas source. Further, the mass filter operates by first ionizing a common source and then can only separate a few metallic species from the source. The species that are separated are limited by the composition of the source. Thus, there is a need for a high brightness ion source that can rapidly switch between multiple different ion gas species. Moreover, there is a need for a system that enables the user to selectively provide gases of different ion species for performing different treatments of a specimen such as milling, etching, deposition and imaging, without requiring replacing the source.